Because solar cells provide a sustainable energy resource, their use is rapidly expanding. Solar cells are typically categorized into two types based on the light absorbing material used, i.e., bulk or wafer-based solar cells and thin film solar cells.
Monocrystalline silicon (c-Si), poly- or multi-crystalline silicon (poly-Si or mc-Si) and ribbon silicon are the materials used most commonly in forming the more traditional wafer-based solar cells. Solar cell modules derived from wafer-based solar cells often comprise a series of self-supporting wafers (or cells) that are soldered together. The wafers generally have a thickness of between about 180 and about 240 μm. Such a panel of solar cells is called a solar cell layer and it may further comprise electrical wirings such as cross ribbons connecting the individual cell units and bus bars having one end connected to the cells and the other exiting the module. The solar cell layer is then further laminated to encapsulant layer(s) and protective layer(s) to form a weather resistant module that may be used for at least 20 years. In general, a solar cell module derived from wafer-based solar cell(s) comprises, in order of position from the front sun-facing side to the back non-sun-facing side: (1) an incident layer (or front sheet), (2) a front encapsulant layer, (3) a solar cell layer, (4) a back encapsulant layer, and (5) a backing layer (or backsheet). In such modules, it is essential that the materials positioned to the sun-facing side of the solar cell layer (i.e., the incident layer and the front encapsulant layer) have good transparency to allow sufficient sunlight reaching the solar cells. In addition, some modules may comprise bifacial solar cells, where the solar cells are able to generate electrical power by receiving sunlight directly reaching the sun-facing side thereof and by receiving sunlight that is reflected back to the non-sun-facing side thereof. In such modules it is essential that all the materials surrounding the solar cell layer be sufficiently transparent.
The increasingly important alternative thin film solar cells are commonly formed from materials that include amorphous silicon (a-Si), microcrystalline silicon (μc-Si), cadmium telluride (CdTe), copper indium selenide (CuInSe2 or CIS), copper indium/gallium diselenide (CuInxGa(1-x)Se2 or CIGS), light absorbing dyes, and organic semiconductors. By way of example, thin film solar cells are disclosed in e.g., U.S. Pat. Nos. 5,507,881; 5,512,107; 5,948,176; 5,994,163; 6,040,521; 6,137,048; and 6,258,620 and U.S. Patent Publication Nos. 20070298590; 20070281090; 20070240759; 20070232057; 20070238285; 20070227578; 20070209699; and 20070079866. Thin film solar cells with a typical thickness of less than 2 μm are produced by depositing the semiconductor layers onto a superstrate or substrate formed of glass or a flexible film. During manufacture, it is common to include a laser scribing sequence that enables the adjacent cells to be directly interconnected in series, with no need for further solder connections between cells. As with wafer cells, the solar cell layer may further comprise electrical wirings such as cross ribbons and bus bars. Similarly, the thin film solar cells are further laminated to other encapsulant and protective layers to produce a weather resistant and environmentally robust module. Depending on the sequence in which the multi-layer deposition is carried out, the thin film solar cells may be deposited on a superstrate that ultimately serves as the incident layer in the final module, or the cells may be deposited on a substrate that ends up serving as the backing layer in the final module. Therefore, a solar cell module derived from thin film solar cells may have one of two types of construction. The first type includes, in order of position from the front sun-facing side to the back non-sun-facing side, (1) a solar cell layer comprising a superstrate and a layer of thin film solar cell(s) deposited thereon at the non-sun-facing side, (2) a (back) encapsulant layer, and (3) a backing layer. The second type may include, in order of position from the front sun-facing side to the back non-sun-facing side, (1) an incident layer, (2) a (front) encapsulant layer, (3) a solar cell layer comprising a layer of thin film solar cell(s) deposited on a substrate at the sun-facing side thereof.
The encapsulant layers used in solar cell modules are designed to encapsulate and protect the fragile solar cells. Suitable polymer materials for solar cell encapsulant layers typically possess a combination of characteristics such as high impact resistance, high penetration resistance, good ultraviolet (UV) light resistance, good long term thermal stability, adequate adhesion strength to glass and other rigid polymeric sheets, high moisture resistance, and good long term weatherability.
Ionomers are copolymers produced by partially or fully neutralizing the carboxylic acid groups of precursor or parent polymers that are acid copolymers usually comprising copolymerized residues of α-olefins and α,β-ethylenically unsaturated carboxylic acids. In recent years, certain types of ionomers have been used as solar cell encapsulant materials. See, e.g., U.S. Pat. Nos. 5,476,553; 5,478,402; 5,733,382; 5,741,370; 5,762,720; 5,986,203; 6,114,046; 6,187,448; 6,353,042; 6,320,116; and 6,660,930, and U.S. Patent Publication Nos. 2003/0000568 and 2005/0279401. More recently, ionomers with excellent optical properties have also been explored as solar cell encapsulant materials, such as those disclosed in Applicants' co-pending U.S. Patent Appln. Publn. No. 2010/0108126, by Pesek et al. entitled SOLAR CELLS MODULES COMPRISING LOW HAZE ENCAPSULANTS. However, there is still a need to develop an ionomeric encapsulant material having not only good optical properties, but also improved moisture resistance.